Insulated gate semiconductor devices such as an insulated gate bipolar transistor (IGBT) and a metal oxide semiconductor field effect transistor (MOSFET) have been widely used in power electronics equipment as switching elements for controlling the power supplied to loads such as a motor. Power loss in the switching elements is preferably less in view of energy saving. Indicators representing g this loss include ON resistance. The ON resistance represents a drain-source electrical resistance when the MOSFET is turned ON. The switching elements suitable for reducing the ON resistance include trench-gate MOSFETs each with a gate electrode embedded in a semiconductor layer. The trench-gate MOSFETs can have channel width densities higher than those of normal planar MOSFETs. Thus, the ON resistance per unit area can be reduced.
Further attention is being given to wide-bandgap semiconductors such as silicon carbide (SiC) as semiconductor materials for the next-generation switching elements. Particularly, its application to a technical field that deals with voltages as high as or higher than 1 kV is viewed as promising. Examples of the wide-bandgap semiconductors include a gallium nitride (GaN) based material and diamond as well as the SiC.
The switching elements are used in, for example, inverter circuits. In order to miniaturize such circuits, increasing the working frequency, that is, accelerating the switching elements is a must. Operation speed of SiC-MOSFETs can be several times higher than that of SiC-IGBTs that have been conventionally widely used. Thus, the wide-bandgap semiconductors are viewed as promising also from this viewpoint. When a semiconductor material having a hexagonal crystal structure, for example, SiC is applied to the trench-gate MOSFETs, a direction of the current path preferably coincides with an a-axis direction with higher carrier mobility. This will expectedly bring a substantial decrease in the ON resistance.
However, the trench-gate MOSFETs for controlling power have a problem with a gate oxide film susceptible to breakage due to the electric field concentration at the bottom of trenches. When the gate oxide film breaks down, the element fails to function as an MOSFET. Thus, techniques for avoiding the electric field concentration at the bottom of trenches in a trench-gate MOSFET have been studied. Particularly, a technique for forming, at the bottom of a trench, a protective diffusion layer with a conductivity type opposite to that of a substrate is well-known. This technique is effective at relaxing the electric field concentration but is insufficient from the viewpoint of its switching. This will be described below.
When high voltages are shut off through switching an MOSFET from an ON state to an OFF state, a depletion layer extending between the protective diffusion layer and the substrate blocks the current path. Conversely, in switching the MOSFET from the OFF state to the ON state, a current path is opened by shrinking the depletion layer. The response speed of the depletion layer in this switching is controlled by the lifetime of minority carriers. Since this time is longer than the switching time, simply disposing the protective diffusion layer does not allow for sufficiently high switching speed.
Patent Document 1 describes electrically connecting a protective diffusion layer to a source electrode by connecting the protective diffusion layer to a base region along trenches to increase the switching speed. Here, the response speed of a depletion layer is determined not by the lifetime of minority carriers but by the time until the minority carriers e extracted by the source electrode. Since this time is shorter than the lifetime of the minority carriers, the technique according to Patent Document 1 produces an advantage of increasing the switching speed. However, the time to extract the minority carriers depends on electrical resistance from the protective diffusion layer to the source electrode. Since a current path particularly from the protective diffusion layer to the base region is narrow under this technique, the resistance increases. Thus, increase in the switching speed may be insufficient under this technique.
Patent Document 2 describes thinning out a part of cells included in an MOSFET and connecting a protective diffusion layer to a source electrode through the thinned-out part. Since a current path from the protective diffusion layer to the source electrode is wider under this technique, this technique produces an advantage higher than that of Patent Document 1.
Patent Document 3 describes a technique aiming at highly integrating semiconductor devices each with both a transistor and a diode. In one example of this technique, contact holes are formed in and along striped trenches each with a gate electrode. Consequently, air anode region of a diode is connected to a source electrode of an MOSFET.